Fluoroelastomers are widely used as materials for seals and hoses in the automotive or aircraft industry because of their elastomeric properties and resistance to heat and chemicals, in particular fuel fumes. In many such applications it is desired that the fluoroelastomers retain their elastomeric properties over a wide temperature range to guarantee their sealing properties. In particular when used in aircrafts, watercrafts or motor vehicles in general, fluoroelastomers are desired that retain their elastomeric properties also at temperatures below −50° C. or even below −70° C. More eminent is the need for materials Therefore, fluoroelastomers having very low glass transition temperatures (Tg) are desired. A particular desire for materials having a low glass transition temperature exists in the field of cryogenic applications like the production and storage of liquefied gases.
A wide range of fluoroelastomers containing copolymers of tetrafluoroethylene, or copolymers of vinylidenefluoride are known and commercially available (compare A van Cleef, in Modern Fluoropolymers, John Scheirs ed., John Wiley & Sons, 1997, pages 597-613).
Fluoroelastomer compositions of high chemical and temperature resistance and good mechanical properties may be prepared by curing a system comprising copolymers of vinylidene fluoride and/or tetrafluoroethylene with perfluorovinyl ethers and cure site monomers. Typically, glass transition temperatures of about −30° C. can be obtained with such systems, as described, for example in European Patent Number 1 829 905 B1. However, the manufacturing costs of perfluorovinyl ethers are comparatively high.
Therefore, there has been a desire to provide fluoroelastomers having low glass transition temperatures to allow for their application as seals in low temperature applications. Therefore, there has been a need to provide fluoropolymers having a glass transition temperature below −60° C., preferably below −80° C. or even lower. Favourably, the materials are elastomers. Desirably, such materials can be prepared at low costs.
Surprisingly, it has now been found that fluoropolymers having very low glass transition temperatures and mechanical properties that make them suitable as elastomers can be made by curing a nitrile-functional low molecular weight fluoropolyether with a curing agent under the formation of triazine group containing linkages. While perfluoropolyethers have been known in the fluoropolymer industry they have been widely used as lubricants or liquid seals due to the fact that they tend to be liquids. The use of perfluoropolyether-based polytriazines as a raw material for making elastomers alongside the known application as lubricants was contemplated in U.S. Pat. No. 5,693,748. Fluoropolyether polytriazines carrying pendant nitrile groups were cured using ammonia as curing catalyst. The resulting polymer was reported to have a Tg in the range of −45° C. However, the cured polymers were reported to be brittle when the polymer unit bearing the pendant nitrile group had a molecular weight of less than about 25,000 g/mole per nitrile group. Such materials were reported to be not suitable as elastomers.